1. Field of the Invention
This invention relates to an air refractometer able to measure the refractive index of air with high accuracy.
2. Description of the Prior Art
Recent advances in scientific and industrial technology have created a need for high-accuracy length measurement. Optical methods are useful for noncontact and high-accuracy measurements because they achieve high-accuracy measurement without touching the objects between which the measurement is made. The interferometric method is particularly well adapted for measuring lengths with high relative accuracy. It has a problem, however, in that the optical wavelengths are affected on the 10.sup.-6 order by the refractive index of the air in which the measurement is conducted.
For overcoming this problem, the inventors previously proposed a two-color method for measuring geometric lengths, specifically a method of measurement using two laser beams of different colors, i.e. different wavelengths, as the light sources of a length-measuring interferometer, thus enabling length values optically measured by optical interference to be corrected in real time for the refractive index of the surrounding air.
In the length-measuring interferometer based on this two-color method a laser beam is directed onto a nonlinear crystal which, as a result, produces a second-harmonic wave. The second-harmonic wave is separated out, and the fundamental wave and the second-harmonic wave are separately used to generate interference fringes. The difference in the number of interference fringes is then determined. If the wavelength of the fundamental laser beam is defined as .lambda..sub.1, the wavelength of the second-harmonic wave as .lambda..sub.2, the length values optically measured using the fundamental laser beam and the second-harmonic wave as L.sub.1 and L.sub.2, respectively, and the indices of refraction of the air at .lambda..sub.1 and .lambda..sub.2 as n.sub.1 and n.sub.2, respectively then the refractive index n.sub.2 of the air with respect to the second-harmonic wave can be expressed as EQU n.sub.2 =1+(L.sub.1 -L.sub.2).A/L.sub.x ( 1)
In this equation, L.sub.x is the value of the measured interval and since an approximate value suffices can be replaced with the measured length value L.sub.1 or L.sub.2. "A" is a coefficient and, as is well known, is a constant given by {(n.sub.02 -1)/(n.sub.02 -n.sub.01)}, where n.sub.01 and n.sub.02 are the refractive indices of air under standard conditions. The value of the coefficient A is dependent on the two wavelengths used and has a value in the range of several tens to several hundreds. As a result, the resolution of the air refractive index measurement by the two-color method employed by the length-measuring interferometer is poor. It is therefore difficult to achieve improved measurement accuracy by optical interference in the relatively short length range between several tens and several hundreds of centimeters.
Moreover, since the conventional length-measuring interferometer using the two-color method requires the interference fringes formed using light of two wavelengths to be separately detected and counted photoelectrically, it not only has poor fringe measurement resolution but also requires a complex measurement systems owing to its need to employ highly coherent light sources etc.
The object of the invention is to overcome the drawbacks of the conventional two-color length-measuring interferometer by providing an air refractometer of simple configuration that is able to measure the index of refraction of air with high accuracy.